fix(coverage): parse case subjects before in

[Chemaclass]

Background

Follow-up to #1341. In case in in ..., the coverage scanner treated the first in, which is the case subject, as the header delimiter. Inside a compact nested case in a substitution, that unbalanced scanner state and could mark a skipped child command as covered.

Changes

• Separate the case phase awaiting its subject from the phase awaiting the in delimiter in both Bash and awk scanners.
• Add Bash/awk parity and coverage-isolation regressions for a subject named in in a compact nested case.
• Clarify that multiline propagation covers executable lines across the physical span, while parent hits do not cover child substitution commands.

Validation

• 60 focused tests and 123 assertions passed.
• ShellCheck and Bash syntax checks passed.
• make lint and git diff --check passed.

Checklist

• I updated the CHANGELOG.md to reflect the fix
• I updated the documentation to reflect the behavior

Refs #1338